This invention relates to the field of freeform fabrication, and more specifically is directed to the fabrication of three-dimensional objects by selective laser sintering utilizing an improved thermal sensing system to ensure accurate temperature sensings of the powder part bed.
The field of freeform fabrication of parts has, in recent years, made significant improvements in providing high strength, high density parts for use in the design and pilot production of many useful articles. Freeform fabrication generally refers to the manufacture of articles directly from computer-aided-design (CAD) databases in an automated fashion, rather than by conventional machining of prototype articles according to engineering drawings. As a result, the time required to produce prototype parts from engineering designs has been reduced from several weeks to a matter of a few hours.
By way of background, an example of a freeform fabrication technology is the selective laser sintering process practiced in systems available from 3D Systems, Inc., in which articles are produced from a laser-fusible powder in layer-wise fashion. According to this process, a thin layer of powder is dispensed and then fused, melted, or sintered, by laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. Conventional selective laser sintering systems, such as the Vanguard system available from 3D Systems, Inc., position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled, in combination with modulation of the laser itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. The computer based control system is programmed with information indicative of the desired boundaries of a plurality of cross sections of the part to be produced. The laser may be scanned across the powder in raster fashion, with modulation of the laser affected in combination therewith, or the laser may be directed in vector fashion. In some applications, cross-sections of articles are formed in a powder layer by fusing powder along the outline of the cross-section in vector fashion either before or after a raster scan that “fills” the area within the vector-drawn outline. In any case, after the selective fusing of powder in a given layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers as appropriate for the article, until the article is complete.
Detailed description of the selective laser sintering technology may be found in U.S. Pat. Nos. 4,863,538; 5,132,143; and 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 to Housholder, all hereby incorporated by reference.
The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including polystyrene, some nylons, other plastics, and composite materials such as polymer coated metals and ceramics. Polystyrene parts may be used in the generation of tooling by way of the well-known “lost wax” process. In addition, selective laser sintering may be used for the direct fabrication of molds from a CAD database representation of the object to be molded in the fabricated molds; in this case, computer operations will “invert” the CAD database representation of the object to be formed, to directly form the negative molds from the powder.
Current commercial laser sintering systems, such as those sold by 3D Systems, Inc. of Valencia, Calif., utilize dual piston cartridge feed systems with a counter-rotating roller and an infrared sensor or pyrometer to measure the thermal conditions in the process chamber and the powder bed.
Although laser sintering systems have proven to be very effective in delivering both powder and thermal energy in a precise and efficient way, the process is thermally based and requires accurate sensings of the powder temperature in the process chamber. The basic problem with attempting to accurately sense the temperature of the powder in a laser sintering system is the fact that contact measurements cannot be made of the powder since powder is constantly in flux during the laser sintering process. Additionally contact sensors are slow, position sensitive, and prone to being disturbed or jarred by bumping. Because of this and since there is no way to ensure consistent contact with the powder without interfering with the powder's movement and hence reducing the accuracy of the measurement, IR sensors have been utilized to attempt to accurately determine the temperature of the powder within the process chamber.
However, the use of a single infrared (IR) sensor focused on one spot on the target surface has some known limitations. These include the lack of a uniform temperature across the entire target surface, possible thermal gradients from front-to-back and from side-to-side of the process chamber and part powder bed, and the fact that the recently fused part in the system is hotter than the surrounding powder. Recognizing these and other limitations, investigators have proposed other approaches to temperature control in laser sintering systems.
One approach has been to address one aspect of temperature control based on an optics and scanning system that detects the temperature of the powder at a detection point near the sintering location and uses that information to modify the laser power and/or modify the temperature of the surrounding powder by use of a traveling defocused laser beam. In this approach and others similar to it, the control is achieved by control of the laser beam power and not by control of a radiant heater. This approach suffers from the required sophistication and expense of the optics system, as well as issues concerning the quality of the radiated temperature signal from the powder as different powders are employed.
Another approach used a machine vision system (a CCD camera) to focus on the target surface of a laser sintering process and measured gray scale color variation of the surface to calculate temperature and modify laser power to maintain consistent part quality. This approach resulted in a lower cost, simpler implementation, but was still based on an average temperature value measured by the camera system.
Still another approach proposes measuring temperatures all across the target surface and making both global (radiant heater) and local (laser) adjustments to the heat input in order to maintain uniform temperatures through the use of a vision system employing an infrared camera to obtain the actual temperatures in the region of the part being produced.
Any system utilizing optics to perform sensing has the inherent problem that the optics degrade over time, thereby decreasing the transmission of signals because of degradation of the electronics, build-up on the optics of absorptive components in the gas within the process chamber or from damage to the optics. Optics degradation can result in the melt down or delamination of parts being fabricated during a build process because of inaccurately low sensor readings that call for increase in heat from the IR heaters until the temperature of the powder being exposed by the laser exceeds its melting point. Alternatively, current systems estimate the optics degradation and attempt to compensate by implementing a temperature ramping downwardly to avoid excess heating. This ramping can cause too low of a part powder bed temperature that results in part distortion, such as “potato chip” type of curling. Both of these phenomena destroy the build.
Thus a need exists for a temperature control scheme for laser sintering that is not limited by the degradation over time of the optics utilized to perform temperature sensings and is able to obtain accurate temperature sensings of the powder in the part bed and the powder feed bed areas within the process chamber of a laser sintering system.